Verbal forms from the root KBS and the expression amr al-kabs indicate: (1) the intercalation and the use of the leap year in the Christian calendars (Biruni, Āṯār, pp. 32-33, p. 241); (2) the embolismal month of Jewish and Indian calendars; (3) the intercalation of the pre-Islamic Arab calendar calculated by a hereditary group termed qalāmes (Biruni, Tafhim, pp. 224-25; on the epithet, cf. “al-Ḳalammas,” in EI² IV, p. 472); (4) the periodical intercalation of a day in the Islamic lunar calendar (Biruni, Tafhim, p. 223); (5) the 5 days of Epagomenae of the Iranian and Coptic calendar (Biruni, Qānun, pp. 74-76; Mollā Moẓaffar, Šarḥ-e bist bāb, bāb 2, section 3), in this way being in Iranian context a synonym of ḵamsa-ye mostaraqa and andargāh; (6) several readjustments that occurred in the Iranian solar calendar (Nowruznāma, pp. 11-12) up to the kabisa-ye malekšāhi, that is,the leap of 18 days ordered by the Saljuq sultan Malekšāh (r. 1072-92) to initiate the Jalāli era (Mollā Moẓaffar, Šarḥ-e bist bāb, bāb 2, section 4; see “The Jalali calendar” under CALENDARS ii). This last usage relates particularly to the readjustments for shifting Andargāh along the year of the “Zoroastrian” calendar, but it does not indicate the deplacement of Andargāhthat occurred in the Iranian Yazdegerdi solar calendar in 1006 CE (Biruni, Qānun, p. 76; the same Biruni, Qānun, p. 142, referring to the reform proposed by the Abbasid caliph, states that “some people call the kabisa of al-Moʿtażed [r. 892-902] the kabisa of the Persians”). (7) The term is also applied to the calendar change performed by Parsi Indian communities in the 12th century (so among modern scholars; Coorlawalla, 1918).

The usual translation of kabisa as “intercalation,” that is, insertion of day(s) tout court, is therefore inadequate. Muslim scholars giving notices on the kabisa of the Persians were conditioned by two axioms: (1) the idea of Arabic kabisa (on which see A. Moberg, “Nasīʾ,” in EI² VII, 1993, pp. 977-78) as duplication in turn of all months (Biruni, Āṯār, pp. 62-63); (2) the Zoroastrian religious unacceptability of forced insertion of one day (cf. Biruni, Tafhim, p. 222). From these, perhaps, derives the theory of an entire one-month insertion every 120 years duplicating the whole of the months in turn. The first source dealing with the kabisa of the Persians in these terms, along with the shifting of Andargāh,is the Zij al-jāmeʿ by Kušyār b. Labbān al-Jili (Ideler, pp. 547-48 tr.; pp. 624-25 text). This idea has been subsequently accepted by many astronomers, among them Biruni (Āṯār, p. 11, p. 44). This pure mathematical-astronomical theory of intercalations is devoid of documentary evidence, but attracted the mathematicians in particular (de Blois, p. 40). In modern times it has been developed, firstly by A. von Gutschmid (pp. 1-9), into the theory of the double Iranian calendar (“religious” and “civil”; Taqizāda, particularly pp. 13-16, 231).

Some sources later than Kušyār and less exploited deal with practices of a different kind, which possibly clarify the question. While the idea and the formula of an ‘intercalary’ turn persist, they refer, not to any insertion, but to mere shifting of Andargāh to serve as a sort of indication of imminent Nowruz, the beginning of the year. According to the Zij al-mofrad (11th century) by Abu Jaʿfar Moḥammad b. Ayub al-Ḥāseb al-Ṭabari (Ms. O.1.10 Browne of the Cambridge University Library, fol. 3a): “The calculation (šomār) of kabisa by the Persians in antiquity was a month in every 120 years, and now that practice has been left behind. . . . Their practice was the following: when the Sun reached the First Point of Aries in the first month of Dey, they called this month Āḏar-māh and Ābān the month of Āḏar. And the five stolen days [scil. Andargāh] were counted at the end of [this] month of Ābān. . . .” For this author the kabisa of the Persians implied only a shifting of Andargāh along with a ‘renaming’ of all the months (Cristoforetti, pp. 46-47). According to Zakariyāʾ al-Qazvini (ʿAjāyeb al-maḵluqāt,p. 82): “There exists divergence on this, some people asserting that the ḵamsa al-aḵira [scil. Andargāh] pertains to abān-māh and some people asserting that the ḵamsa al-aḵira pertains to aḏar-māh.”

A precise echo of this “practice” may be found in Montahā al-edrāk fi taqāsim al-aflāk by Abu Bakr Moḥammad b. Aḥmad al-Ṯābeti al-Ḵaraqi (Ms. Or. 110 of the Biblioteca Medicea Laurenziana in Florence, fol. 93b): “The Saturday, 12th of the month of Rabiʿ II in the year [52]5, year 500 in the era of Yazdejerd, the turn of the kabisa returned to the month of Ordibehešt, and therefore we held kabisa in the month of Farvardin (fa-kabasnā farvardin-māh), and we added the 5 epagomenal days to its last days, and so its days numbered thirty-five.” The stated operation explicitly relates to the month of Farvardin, which becomes “stuffed” (makbus). So at this time (12 Rabiʿ II 525/14 March 1131/1 Ordibehešt 500 Yazdegerdi = vernal equinox) Andargāh was shifted to the end of Farvardin. The same information is to be found as a prediction of the next calendar adjustment in Šahmardān b. Abe’l-Ḵayr Rāzi’s Rawżat al-monajjemin (ms. Or. 4° 848, mutilated, of the Staatsbibliothek in Berlin; cf. Taqizāda, p. 20, n. 41, and p. 234).

The persistence of this practice is confirmed by observations in the modern calendars of at least two regions of the Iranian world: in the Pamir region (in many centers of Badakhshan such as Āvšān and Šoḡnān and surrounding areas) there are three Nowruzcoexisting today: the ‘regular’ Nowrūz of 1 Farvardin, one preceding 17-18 February (end of Bahman), and one following Ordibehešt (Karamšāhyef, pp. 687-88). For Iran, we have the following report on the usages of the area of Ṭalaqān (Gilān province): “Panje days begin from 25th of Farvardin of every year called ‘Panje Pitok’ in Taleghan and its surrounding villages. Like people of other cities, people of Taleghan calculated every month for thirty days which totally became 360 days in every year. By this calculation, there were five days that belonged to none of the months” (Vakilian, p. 202).

Probably around the 20s of the 12th century—when, in a calendrical system which still remained solar and vague, Nowruz was behind by roughly a month with respect to the beginning of Spring—Andargāh and Nowruz were displaced by a month, thus giving birth to a calendar with Nowruz on the first of Ordibehešt and Andargāh immediately before, that is, at the end of Farvardin (putting into practice the ‘intercalary’ method mentioned in Montahā al-edrāk). We can see an indication of another occurrence of a shift of Andargāh, to a position between Dey and Bahman, in the five-day discrepancy between Bar-sada and Sada festivals (Cristoforetti, p. 56). But we also have various hints at a popular custom (šab-e esfand) of shifting Andargāh to a position between Bahman and Isfand (idem, p. 49).

The Parsi Zoroastrians appear to have applied a kind of kabisa similar to the one described in the Zij al-mofrad, paraphrasing which, one could describe it in the following manner: when the Sun reached the First Point of Aries on the first of Ordibehešt, they called this month Farvardin, and Farvardin became Esfandārmaḏ. In spite of a prevailing hostility to changes in the calendar in Zoroastrian quarters, rather than think of the only effective insertion of a supplementary month in the entire history of the Iranian calendar (Boyce, p. 20; de Blois, p. 50), it is preferable to conceive of it as something that is amply testified as widespread throughout the Iranian world.

In conclusion, the mechanism defined by Islamic astronomers as kabisa was extremely simple, but gave rise to various confusions. There was no true intercalation, but rather a periodic readjustment. This practice is attested on the popular (provincial) level, but was officially enacted only once, in the Sasanian era, when Andargāh seems to have moved to a place following Ābān. (Baliński, p. 101, hypothesizes two displacements of Andargāh: one in the Parthian and the other in the Sasanian age.) The eventual renaming of the months illustrated by Zij al-mofrad possibly creates another parallel calendar, thus casting light on the issue of the ‘double’ Iranian calendar first hypothesized by A. von Gutschmid and later analyzed by Ḥ. Taqizāda. At the beginning of the 11th century, something similar was done, displacing Andargāh until after Esfandārmaḏ, and during the whole Islamic age we can observe similar, although less thoroughly welcomed, practices.

S. Cristoforetti, “The ‘Hall of the Ambassadors’ Paintings in the Frame of the Calendrical System of the Iranian World,” in M. Compareti and S. Cristoforetti, New Elements on the Chinese Scene in the “Hall of the Ambassadors” at Afrāsyāb and a Reconsideration of “Zoroastrian” Calendar (Eurasiatica n. 78, Dept. of Eurasian Studies publications, Ca’ Foscari University, Venice), Part II, Venezia, 2007, pp. 33-71.